I would say that the record of life on this planet is that if there is a niche to fill that
something will take the offer up -- look at all the extreme ecologies of bacteria that keep showing up.
Consider that there is absolutely no barrier to plant life migrating from the sea to the land, as a gradual progression from tidal to estuary to mudflat etcetera is available ... any mutation that allows a first pioneer to survive in the slightly different environment will succeed massivly (no competition and no predators).
For the same reason any form of life that is a predator of the plant colonists will have the same massive success for the first pioneers, no matter how sluggish.
and they would, of course, be followed by predators of their own, and the race would be on ....